Conventionally, electric double-layer capacitors attract the attention for their high withstand voltage, large capacity, and furthermore, high reliability in rapid charge and discharge.
Moreover, an electrochemical capacitor having an energy density as high as 4 V upper voltage realized by a carbon material comprising a negative electrode occluding a lithium ion (Li+) has been proposed as a next generation version of the electric double-layer capacitors.
Conventionally, improvement of the cycle characteristics at the time of rapid charge and discharge has been executed by reduction of the production amount of hydrogen fluoride (HF) produced from an electrolyte in a secondary power source using a lithium ion at the time of charge and discharge including the case of the electrochemical capacitor. Specifically, an electrolyte including as an electrolyte salt fluoro alkyl phosphate alone (LiPF6−n (R)n (wherein n is an integer from 1 to 3, and R is CF3 or C2F5)), or a mixture in a 1:1 ratio of fluoro alkyl phosphate and LiPF6 or LiBF4 is used (see patent literature 1).
However, according to the secondary power source using the electrolyte salt as a mixture in the ratio mentioned above, since LiPF6 or LiBF4 included therein easily reacts with a minute amount of moisture content in a cell so as to generate HF to such an extent of deteriorating the cycle characteristics.
Then, the produced HF destroys a protection coating film including lithium formed on the surface of the carbon material of the negative electrode. Since the lithium ion occluded in the negative electrode is consumed for the repair of the protection coating film destroyed by the HF, the lithium ion occluded to the negative electrode is reduced so that the electric potential of the negative electrode is increased for causing decline of the withstand voltage of the electrochemical capacitor.
Furthermore, as to a portion without being affected by the destruction by HF in the protection coating film, the film becomes thicker at the time of repairing the broken portion so as to increase the internal resistance of the electrochemical capacitor.
That is, the electrolyte of the mixing ratio mentioned above provides the HF production restraining effect with fluoro alkyl phosphate insufficiently.
On the contrary, an electrolyte using as the electrolyte salt only fluoro alkyl phosphate may be used for an electrochemical capacitor.
In this case, since the fluoro alkyl phosphate has characteristics hardly generating HF even in the case of being decomposed in the electrolyte, HF generation is restrained.
However, in the case of carrying out charge and discharge of the electrochemical capacitor with an excessive voltage of about 4.5 V applied at a high temperature of about 80° C., oxidation reaction of the electrolyte is generated at a positive electrode and at the same time, lithium ion is more occluded inside the negative electrode in the amount according to the charge amount of the positive electrode reaction.
Thereby, the electric potential of the negative electrode is lowered excessively. If charge and discharge operations are repeated in this state, there is a risk of deposition of the lithium ion occluded excessively as lithium metal on the negative electrode surface.
Then, the deposited lithium may break and short-circuit a separator in the vicinity of the negative electrode.